Printed circuit board chassis are typically fabricated using stamped and formed sheet metal so that they meet structural requirements while maintaining low tooling costs, reduced design time and reduced delivery time. Stamped sheet metal chassis typically also provide structures compatible with creating proper airflow through a cabinet and minimize wall thicknesses, an important consideration for clearances within densely packed enclosures.
Printed circuit boards themselves are usually densely populated, and are therefore heavy and can easily be damaged when they are inserted into tightly-spaced card racks. This problem is aggravated when individual card guides formed in sheet metal components are set back from the front of the rack due to design constraints such as existing tooling, air management, nut bar locations, and the like.
Stamped sheet metal circuit card racks are nonetheless presently the preferred type of chassis, despite the limitations they impose on the lead-in or starting position for inserting boards. The leading edge of the guide should be gradual and curved, to conform to stamping and forming requirements, however, as noted above, the stamped construction also often dictates that it is recessed from the front of the chassis. A recessed lead-in, however, is not desirable since the distance between the edge of the rack and the entry point of the lead in provides an opportunity for misalignment and card damage. Thus, there is a need to develop improvements in stamped sheet metal circuit card guides, and in particular it would be desirable to improve the lead-in and move the entry point closer to the edge of the enclosure.
A number of prior patents disclose card mounting slots molded in plastic that extend across the full width of the enclosure. For example, U.S. Pat. No. 3,723,823--Lit et al. discloses a printed circuit board guide of molded plastic which, as illustrated in FIG. 2 thereof, has an integral molded lead-in section and a board-retaining section. The disclosed printed circuit board guide extends across the width of the enclosure and is substantially the same length as the width of the board being held in place. U.S. Pat. No. 4,019,099--Calabro discloses a printed circuit board guide in the form of a track mounting device molded from plastic that extends across the width of the enclosure and is substantially the same width as the width of the board that is being retained. The plastic board mounting device is attached to the sheet metal of the electronics enclosure using integral elements in the form of a split post that acts as a fastener. U.S. Pat. No. 4,068,290--Wetherbee discloses a device for use in an enclosure that has full length card slots disposed a fixed distance apart. A disclosed cantilever card guide is affixed to one side wall of the enclosure and also extends across the full width, permitting circuit boards of less than the full height to be mounted in the enclosure.
Others have attempted to create improved card guides using full width metal channels, as shown in U.S. Pat. No. 5,008,778--Peyerl, which discloses a card guide that has a lead-in section, and is adaptable to accommodate either the edge of a circuit board in a groove, or to present an edge that engages a groove in a circuit card. U.S. Pat. No. 4,429,937--Stockmaster discloses a rack formed from structural shapes of metal that has full length card slots individually mounted on the angle pieces that form the frame of the enclosure.
It is also known in the prior art to retain a circuit card only near its edges, i.e., the channel in which the card rests does not extend across the full width of the enclosure. These types of card guides are not used in conjunction with another card guide, such as a sheet metal card guide, but function as both the lead-in and the card retention structure. U.S. Pat. No. 4,508,228--Erlam discloses modular card frames adaptable between two different heights. The card locations are defined by end pieces that are connected by a bar, and the end pieces are attached to the assembly by pegs that extend from cross rails. U.S. Pat. No. 4,167,032--Scagnelli discloses edge guides that are a series of spring fingers that both guide and support the circuit board. U.S. Pat. No. 4,399,485--Wright et al. discloses edge guides that are essentially a flat bar with grooves running across the width of the bar.
A lead-in guide placed in front of a card guide is disclosed in U.S. Pat. No. 4,075,683--Johansson et al. The disclosed design provides a molded lead-in portion that is slid into a specially designed bar by the cooperation of a molded flange on the lead-in guide and a groove in the bar.
The molded card slots, edge guides, lead-in guides and other alternate construction techniques disclosed in the prior art discussed immediately above all require greater tooling costs as well as increased material wall thicknesses. As noted above, increasing wall thicknesses is not desirable since the increased chassis material reduces the available space within the enclosure. Thus, there remains a need for improvements in the lead in section of stamped metal circuit card guides.